The market for low cost and sensitive radio frequency receivers is enormous and current technology solutions are fairly expensive. Low frequency (LF) receivers are used for self-setting clocks that receive the National Institute of Standards and Technology (NIST) WWVB time information and are used in high-end consumer and industrial applications. The cost of such receivers keep this technology out of the mass markets such as energy metering and lower end consumer electronics.
Various radio frequencies are used to transmit this time standard. The NIST radio station WWVB transmits at a very low frequency (VLF) of 60 kHz and effectively distributes standard time information to better than one second throughout the North American continent. Other VLF time standard transmitting sites are in the far east—JJY (Japan) and Europe—MSF (UK). NIST WWV also transmits time information in the high frequency (HF) radio spectrum.
The NIST radio stations (e.g., WWV, WWVH, WWVB) are continuously being used for both precise frequency and time calibration, The demand for precise frequency and time calibration is constantly growing as manufacturers continue to create new, lower cost products, in an effort to place “Atomic Time” in every home and office. However, acceptance of highly accurate and automatically set time appliances is greatly dependent upon cost and ease in implementation. Integrated circuit technologies have reduced the cost of time measurement, recording and display systems, e.g., digital clocks, parking meters, etc. However, complex and expensive receiving equipment is presently used to receive the time signals from the NIST radio stations. Apparatus and systems requiring accurate time information may be for example, but not limited to, clocks, time of use utility meters, traffic lights; bus, train and plane scheduling apparatus; speed measuring instruments used in combination with global positioning satellite (GPS) devices, timers, parking meters, and the like.
Existing super-regenerative receivers use controlled quenching that introduce unwanted noise into the resonant tank circuit of the super-regenerative receiver, thus reducing the ultimate sensitivity of the receiver. In addition, the sensitivity of the super-regenerative receiver is non-linear due to the controlled quenching. Alternatively, super-regenerative receivers require a large number of active and passive devices to realize a practical and sensitive solution which is both expensive and difficult to implement in an integrated circuit.
In addition, implementation of a low frequency (LF) reception super-regenerative receiver is problematic in that the quench frequency and the carrier frequency are relatively close to one another, leading to difficulties in designing for reduced cross modulation (distortion), and thus unwanted noise. Another problem with LF passive quenching is the large capacitor and inductor values required for implementation, these components take up space and increase cost. Super-regenerative receivers also have the unwanted characteristic that the quenching action radiates unwanted noise and thereby causes other super-regenerative receivers to “hear” one another when placed in close proximity.
Known regenerative receivers change the bias level to obtain the quench action, this results in the effective quality factor (hereinafter “Q”) of the tank circuit to continuously change, and that the Q is low during the critical startup phase of the tank circuit. A high Q is desired at startup when “sampling” the incoming radio signal, having a low Q results in the existing regenerative receivers being noisy because they receive wide bandwidth (low Q tank circuit) noise during startup. Another disadvantage of known regenerative receivers is that with a varying bias, the effective receiver bandwidth (Q) changes with signal strength and thus noise performance can worsen when receiving weak signals.
U.S. Pat. No. 5,630,216 makes use of controlled quenching by means of a decaying current source that is realized by a square wave source and a pulse forming networking consisting of two capacitors where I=C dv/dt. This approach introduces unwanted noise due to the high dv/dt square wave source, the author ties to solve this problem with excessive filtering but the ultimate sensitivity is limited. The solution addresses the noise emission problem by de-coupling the tank and the receiver sections and not limiting by limiting the oscillation amplitude. U.S. Pat. No. 6,035,002 adjusts the biasing level of the receiver tank to alter the start up time of the regenerative oscillator, the time is then compared to a reference signal to determine whether start-up is faster or slower than expected. The solution makes use of a number of active stages and the biasing and control circuitry that induces noise into the receiver and thus reduces the ultimate sensitivity. The solution addresses the noise emission problem by de-coupling the tank and the receiver sections with an active buffer.
Therefore, what is needed is a low cost and sensitive super-regenerative receiver that can be easily fabricated in an integrated circuit.